Conventionally, for an insulated electric wire that is preferably used for wiring of parts for a car such as an automobile and parts for an electrical/electronic appliance, there has been widespread use of an insulated electric wire in which a conductor is covered with a vinyl chloride resin composition to which a halogenous flame retardant is added.
However, this kind of vinyl chloride resin composition includes halogen elements, so that it emits harmful halogenous gas into the atmosphere in case of car fire or at the time of disposing of an electrical/electronic appliance by incineration, causing environmental pollution.
From the view point of reducing loads on the global environment, a so-called non-halogenous flame-retardant composition including an olefin resin that does not emit harmful halogenous gas during combustion to which metal hydroxide such as magnesium hydroxide that is a non-halogenous flame retardant is added has been recently used as an alternative to the vinyl chloride resin composition.
In this kind of flame-retardant composition, a large amount of magnesium hydroxide is added in order to impart sufficient flame retardancy. Synthetic magnesium hydroxide that is synthesized from sea water is comparatively expensive, and therefore, low-price natural magnesium hydroxide has been recently used in order to reduce the production cost.
For example, Patent Literature 1 discloses an insulated electric wire that is covered with a flame-retardant composition prepared by adding hydroxide magnesium that is a flame retardant to a resin or a rubber such as an ethylene-ethyl acrylate copolymer (EEA), polyethylene, and an ethylene propylene rubber.